Vibration Analysis Pinpoints Inadequate Motor Bearing Lubrication

Rolling element bearings
are among the most important components in the vast majority of machines, and
exacting demands are made upon their carrying capacity and reliability. The
continued research and development of rolling bearing technology has enabled
engineers to calculate the life of a bearing with considerable accuracy, thus
enabling bearing life and machine service life to be accurately matched.

Unfortunately, it sometimes
happens that a bearing does not attain its calculated rating life. There are
many reasons for this - heavier loading than had been anticipated, inadequate
or unsuitable lubrication, careless handling, ineffective sealing or fits that
are too tight causing insufficient internal bearing clearances. Each of these
factors produces its own type of damage and leaves its own special imprint on
the bearing.

Vibration
Analysis Detects Inadequate Lubrication in Fan Bearing
In some cases, it is possible to detect the effect of inadequate lubrication
within a rolling element bearing by utilizing vibration analysis as a preventive
maintenance tool. This ability to detect lubrication deficiencies with vibration
analysis was proven at one facility when a noise was heard in an electric fan
motor.

The electric motor is one
of six fan motors installed on a cooling tower at a large refinery in England.
All six are lubricated with Shell Nerita Grease HV semisynthetic high-speed
bearing grease, and are included in the production unit lubrication program.

During the routine scheduled
vibration monitoring activity, this electric motor was observed by the condition
monitoring operative to be emitting an intermittent high-pitched noise from
the nondrive-end (NDE) bearing location, albeit very low in amplitude. Routine
overall vibration readings taken throughout the motor exhibited readings below
1 mm/sec rms and were consistent with previous values.

This prompted further “in-field”
investigation to determine the location and/or cause of this noise. This investigation
was carried out by setting the portable vibration data collector into “analyzer”
mode. With the required collection set and frequency max selected, real-time
data was obtained from both the NDE and drive-end (DE) motor bearing locations
while the motor was at full speed; this data was taken in all three axis (where
access permitted).

Examination of the vibration
spectrum from the NDE bearing revealed, in the vertical direction, a “haystack”
effect in a frequency band between 2 kHz and 3.5 kHz (Figure 1).

Figure 1. Single Spectrum
Plot

Much has been written and
many projects have been undertaken to determine where a haystack would be evident
in terms of frequency band. While it is largely dependent on bearing type, speed,
load, etc., this determination seems to be hit and miss, and experience in detection
and resolution plays an important role.

Based on the author’s nearly
20 years of experience, this type of spectrum is normally attributed to a reduction
in lubrication quality or lubrication effectiveness, resulting in some degree
of metal-to-metal contact within the rolling element bearing. If this condition
is not resolved, accelerated bearing wear occurs, leading to an increase in
operating temperature and ultimately bearing failure.

Once the condition was determined,
a given amount of lubricating grease was applied to this bearing. With the vibration
data collector still set in analyzer mode, the analysts were able to visually
monitor the immediate effect the lubricating grease had on the vibration and
this haystack effect.

After the grease was added
and had been distributed within the bearing, the live spectra indicated a very
slight recurrence of the haystack. It was then decided that a few shots of grease
should be applied. The effect was immediately noticeable with a considerable
reduction in the vibration haystack (Figure 2), which, after some time, did
not return.

Figure 2. Single Spectrum
Plot with Reduced High Frequency Vibration

The reason this particular
motor showed this symptom is unknown, and the actual severity of the bearing
wear/damage is also unknown. Maintenance records show, however, that this motor
continued in full service for another five years without the need for bearing
replacement. Therefore, it is clear that by locating and correcting this lubrication
deficiency, a motor bearing failure due to this condition was prevented.

About the Author
Mr. Stevens started out in the condition monitoring field in the 1980s and has
been fully involved in it ever since. He is a qualified mechanical engineer
and a fellow member of the Institution of Diagnostic Engineers (FIDiagE). His
past employment has included specialist companies such as AV Technology Ltd,
Manchester, England. Mr. Stevens now works as the equipment condition monitoring
(ECM) solutions manager for Shell Services, responsible for solution development
and support to the European arm of the services business.